The invention relates to programmable logic controllers. Specifically, the invention utilizes removable boards inserted into a programmable logic controller to allow the user to manually override specific outputs of the controller.
Prior to the development of solid state switching devices, electro-mechanical relays, timers, counters, and sequencers were used in control panels in various interconnecting sequences to control and automate processes. These panels were unwieldy in size, slow to react, and inflexible if the process being automated was changed. Advances in semi-conductor technology have resulted in the development of programmable logic controllers (PLCs). PLCs are used in any application which has required the automation of a process. According to the National Electrical Manufacturers Association (NEMA):
A PLC is a digitally operated electronic system, designed for use in an industrial environment, which uses a programmable memory for the integral storage of user-oriented instructions for implementing specific functions such as logic, sequencing, timing, counting, and arithmetic to control through digital or analog inputs and outputs, various types of machines or processes. Both the PLC and it""s associated peripherals are designed so that they can be easily integrated into an industrial control system and easily used in all their functions.
National Electrical Manufacturers Association, NEMAIA 2.1 standard, adopted from the International Electrotechnical Commission, IEC 1131 standard, part 1, section 2.50.
PLC""s have a great economic advantage over other controlling methods in that one PLC can be used with a wide range of control systems. To modify the PLC to accommodate a variety of different processes, there is no need to rewire the PLC. All that is necessary is for an operator to program a different set of instructions. This flexibility allows a single PLC model to be bought off the shelf and utilized in a variety of applications, eliminating the need for specialized wiring designs and allowing large numbers of the PLC model to be mass produced, thereby lowering the cost of each unit.
A PLC typically consists of an input section, a logic (or processing) section, an output section, and a power supply. Typically, the power supply is used to provide logic level voltage (around 5-15 VDC) to the logic portion of the PLC. The input section receives electrical signals from various sensor units which are converted to logic signals. Examples of input sensing devices would include: limit switches, toggle switches, selector switches, push button switches and solid state proximity devices. Once an input signal is received by the input section and converted to a logic signal, it is communicated to the logic sections. The logic section then evaluates the input (or multiple inputs) with respect to a pre-programmed series of logic steps to determine if an output signal should be generated. A series of programmable logic steps is used to make this determination. Subsequently, a control signal is sent to the output section which biases a switching mechanism inside the output module (typically a solid state switching device, such as a transistor) into an xe2x80x9cONxe2x80x9d or xe2x80x9cOFFxe2x80x9d position depending on the nature of the output signal. The switching mechanism is used to control a load which is connected to terminals on the output section. This is typically accomplished by providing power to the output module at a xe2x80x9ccommonxe2x80x9d terminal and allowing it to pass through (or preventing it from passing through) the switching device (transistor), thereby enabling (or disabling) the desired load. The load could be almost anything, including lights, motor starters, contactors, and resistive heating elements. Multiple inputs and outputs are provided with an individual PLC, allowing a large number of loads to be controlled by a single PLC.
One problem with current PLC""s is that in certain situations, it may be desirable to manually override the logic portion of the PLC so as to generate an output signal regardless of whether the appropriate input signal has been detected by the PLC (for example when maintenance is being performed on the PLC or the load). This situation arises particularly when the PLC is controlling the environmental conditions of buildings, such as when controlling heating, ventilation and air-conditioning equipment, overhead lighting, and alarm systems. It is desirable to override the control of certain loads while allowing the PLC to maintain control over the other PLC outputs, since it may often be desirable to turn one system off or on while allowing the others to continue to operate. For example, it may be desirable to turn off a heater for maintenance without disabling an alarm system. Maintaining the flexibility to choose which (if any) output has this override feature without losing the ability to manufacture the PLC in the most economical manner is extremely valuable.
Additionally, it may be desirable in some situations to allow for additional or alternate switching mechanisms to be interposed between the controlled load and the switching mechanism of each PLC output. These switches may be necessary to control a variety of loads which require various current and voltage requirements. It would be desirable to install these switching mechanisms without having to purchase a completely different PLC output section containing different switching mechanisms in the output section of the PLC. Previously, to address this problem, an additional switching mechanism (such as a electromechanical relay) was disposed electrically downstream from the output section of the PLC. The additional switching mechanism was not purchased as a package with the PLC but instead was separately engineered, installed and mounted outside the PLC. The output section of the PLC actuated the coil of the electromechanical relay which allowed the making and breaking of the circuit controlling the load through the relay contacts. Installing this additional switch required wiring additional circuit boards or switching mechanisms to the terminals of the output section. The switches were physically mounted to a support structure (such as a din rail) externally from the PLC. This method of adding another switching mechanism has many drawbacks. For example, it required additional designing, it was not modular or xe2x80x9coff the shelfxe2x80x9d and it required additional installation space.
Therefore, a need exists in the art which allows the output section of a PLC to be manually overridden at the installation site of the PLC. Additionally, a need exists in the art for a modular system to provide flexibility to the output modules of PLC""s, allowing the end user to alter the load carrying capacity of the output device, without requiring a complicated installation process.
The invention is an override board releasably mounted in a programmable logic controller. The override board is used to control an individual output of the programmable logic controller. The override board includes a manual switch mounted to a printed circuit board assembly. The manual switch has an xe2x80x9cautoxe2x80x9d position, an xe2x80x9conxe2x80x9d position and an xe2x80x9coffxe2x80x9d position. An output device is also mounted to the printed circuit board assembly and is electrically connected to the manual switch. Placing the manual switch in the xe2x80x9cautoxe2x80x9d position transfers control of the override board to the programmable logic controller. Placing the manual switch in the xe2x80x9conxe2x80x9d position energizes the output device. Placing the manual switch in the xe2x80x9coffxe2x80x9d position de-energizes the output device.
Additionally, a feedback circuit may be provided on the printed circuit board assembly which provides a signal to the programmable logic controller. The feedback signal indicates the position of the manual switch. A light emitting diode may also be mounted to the printed circuit board assembly, providing a visual indication of whether the output device is energized or de-energized.